This invention relates to thyristors and, more particularly, to high current carrying thyristors with pilot devices therein to facilitate rapid device turn on and having gate turnoff capability.
Semiconductor controlled rectifiers are becoming more popular. Circuit design engineers have been developing an ever increasing number of applications for them. Some applications are demanding and require unusual electrical characteristics in the controlled rectifiers to be employed. For example, some applications require extremely high current devices and other applications require devices with very fast response times. Certain applications require a combination of unusual characteristics in a given device. For example, high power applications often require high frequency characteristics also. However, when the structure of controlled rectifiers is altered to provide a given characteristic in the finished devices, other characteristics are sometimes adversely affected. For example, to provide a device with a high current carrying capability the conductive area is generally made larger. Unfortunately, an increase in the size of the current conducting area generally has an adverse effect on device response time. This is true for the following reason. A semiconductor controlled rectifier is typically a four layer device defining three PN junctions. One of the outer layers is a cathode emitter. In general, the conductive area of the controlled rectifier is coextensive with the area of the cathode emitter. However, turnon of the rectifier is possible only at the periphery of the emitter and, often, occurs only at a point or very few points along the periphery. A finite propagation time is required for the remainder of the potentially conductive area to be rendered conductive. In large devices this propogation time can be several hundred microseconds. It will be appreciated that such along turnon time prevents efficient utilization of large devices with high frequency, high power signals. Thus, when a conventional semiconductor controlled rectifier is utilized with high frequency high power devices, full power is sometimes applied more rapidly than full conduction can be achieved. In that event the current density in the area that is conducting can exceed the safe limit for the device in question and lead to device failure. The above is, of course, well known in the prior art. See, for example, the SCR MANUAL, Fifth Edition, copyright by the General Electric Company, 1972.
One solution to the problem of an unacceptably long propagation time has been the interdigitation approach. This is also discussed in the SCR MANUAL. When it is desired to utilize the interdigitation technique, the periphery of the cathode emitter, rather than being a regular shape such as a rectangle or a circle, is provided with a plurality of indentations so that all points of the emitter are relatively near the peripehery. Consequently, propagation time becomes less of a problem.
However, interdigitation has an effect on other device characteristics. For example, it has been found that in order to turn on the entire periphery of the cathode emitter a large triggering current is required. The current required is dependent upon the length of the periphery. It will be appreciated that an interdigitated device includes an extremely long emitter periphery and thus a very high triggering current is required to turn on the entire periphery. For example, in a controlled rectifier capable of conducting a current of 200 amperes, a triggering current in the range of 5 to 10 amperes may be necessary. This, of course, requires that low level triggering signals be substantially amplified in order to be usable. Thus, triggering circuits are rendered quite complex. Failure to simultaneously turn on the entire emitter periphery negates much of what was gained by utilizing an interdigitated structure. Thus, in summary, interdigitation provides a technique for rendering high current semiconductor controlled rectifiers amenable to high frequency operation. However, the resulting devices require substantial triggering currents to operate effectively.
In order to overcome the high triggering current requirements of high current SCRs, self-contained pilot units were incorporated in SCRs in a manner more fully described below. Briefly, a pilot SCR is a controlled rectifier coupled to the source of current to be controlled and the gate of the primary SCR. A small triggering current is sufficient to activate the pilot SCR which in turn supplies a substantial gate current to the primary SCR thus insuring that the entire periphery of the primary SCR emitter is rendered conductive. Thyristors including pilot SCRs have become popular, particularly in high power high current devices.
A characteristic of most semiconductor controlled rectifiers is that once rendered conductive they remain conductive until the current passing through them ceases at which time they become non-conductive and remain so until triggered again. Thus, no sustained triggering signal is necessary if a semiconductor controlled rectifier is to remain in the conductive state for a sustained period of time. Depending upon the use to which the device will be put, this characteristic may be an advantage or a disadvantage. In response to the realization that the "latching" characteristic of SCRs could sometimes be disadvantageous, efforts were made to find a turnoff mechanism independent of the controlled current passing through the SCR.
It was discovered that in some SCRs withdrawing current from the gate will induce a device turnoff. This phenomena is also discussed in the SCR MANUAL. Consequently, some devices were adapted to respond efficiently to turnoff signals on the gate. These devices were generally referred to gate turnoff devices.
Consequently, it would seem that a device with desirable characteristics for certain applications could be fabricated by utilizing a pilot SCR in a gate turnoff device. Such a device would have a high current carrying capability, rapid turnon time and gate turnoff capability. However, for reasons to be discussed more fully below it has not heretofore been possible to effectively incorporate a pilot SCR in a gate turnoff device.
It is, therefore, an object of this invention to provide a thyristor comprising a primary SCR capable of carrying high currents and including a pilot SCR for insuring rapid turnon of the primary SCR with low level gate signals and wherein the primary SCR has gate turnoff capabilities.